Breast carcinoma is the most frequent cancer among women with considerable invasive and metastatic behavior. CCND1, the oncogene encoding cyclin D1, is amplified in a substantial proportion of human cancers. Although cyclin D1 overexpression has been reported in up to 50% of human breast cancers, its prognostic impact on breast carcinoma is still controversial.In this cross-sectional investigation, 89 patients with breast invasive ductal carcinoma enrolled in the study. Tumor tissue samples were stained immunohistochemically for cyclin D1. The marker was semiquantitatively scored using the Allred scoring method and its relationship with ER, PR, and HER2-neu status as well as age, tumor grade and stage was then determined.Cyclin D1 was strong (S), intermediate (I), weak (W), and negative (N) in 19.1%, 44.9%, 14.6%, and 21.3% of the cases, respectively. Estrogen receptor (ER), progesterone receptor (PR), and HER2- neu were positive in 60.7%, 58.4%, and 36% of the cases, respectively. There was a statistically significant reverse relationship between tumor grade and cyclin D1 (P = 0.009). The relationship between cyclin D1 and both hormone receptors was also statistically significant (P = 0.0001). There was no statistically significant relationship between cyclin D1 on one hand and age, stage, and HER2-neu on the other (P > 0.05).The reverse relationship between cyclin D1 overexpression and tumor grade as well as its positive relationship with ER and PR in invasive ductal carcinoma suggest that cyclin D1 may directly or indirectly result in maturation and differentiation of tumor cells.